This invention relates to a process for increasing the rate of zonal concentration and final density of solids in an aqueous dispersion of such solids and to apparatus for the practice of such process.
In many industrial processes, aqueous dispersions of solids are obtained as waste streams. For example, washing of ores, scrubbing of gas streams, precipitation of impurities, sewage disposal, etc., often produce waste streams referred to industrially as tailings, mud, etc. Such waste streams often cannot be conveniently disposed of due to materials handling difficulties or ecological or safety considerations. Therefore, it is a common practice to retain such wastes in large ponds, holding tanks, or other containment means. To the extent the solids in the retained dispersions can be zonaly concentrated by settling, floatation, etc., to provide a relatively solids-free aqueous phase which can be sewered or otherwise disposed of following any necessary filtration or purification procedures, space is provided in the containment means for retention of additional waste. However, in the case of many aqueous dispersions, zonal concentration of the solids by settling takes place at an extremely low rate, if at all. Thus, periodic construction of additional containment means at great difficulty and expense becomes necessary. Numerous attempts have been made at increasing the rate of zonal concentration of solids in such dispersions. However, the use of conventional means such as filtration, settling aids, or the like, is frequently physically impossible or prohibitively expensive. It has been found that laboratory techniques for effecting zonal concentration of solids in aqueous dispersions often cannot be successfully applied to large scale industrial waste retention systems. For example, it is known that applying an electrical potential between electrodes placed in an aqueous dispersion will cause migration of water towards the cathode to effect zonal concentration of solids via electroendosmosis. However, in large systems where the anode and cathode must, as a practical matter, be separated by relatively great distances, large and non-linear voltage drops occur which prevent effective application of the electroendosmotic technique.
It is thus well recognized that practical means for promoting zonal concentration of solids in aqueous dispersions have long been desired by those skilled in the art.